The technical field of the invention relates to injection systems, in particular common-rail injection systems or common-rail injectors having hydraulic transmission. Due to the principles involved, actuation of the pilot valve of the hydraulic transmitter results in a switching leakage.
In this regard FIG. 1 shows a schematic view of a known injector I, with reference to which the switching leakage that occurs due to the principles involved and a conventional approach to a solution for reducing the occurrence of the switching leakage are discussed. The injector I depicted in FIG. 1 has a magnet actuator MA, a high-pressure connection HA for carrying pressurized fuel, a pilot valve SV and a nozzle D by means of which the fuel is injected. A control piston SK is arranged between a valve pin DN and the pilot valve SV which is actuated by means of the magnet actuator MA. When the pilot valve SV is opened by means of the magnet actuator MA, a pressure drop is produced in a control chamber SR containing the control piston SK. The pressure drop occurs in particular because a discharge throttle AD between the control chamber SR and a valve chamber VR containing the pilot valve SV is bigger than a supply throttle ZD which couples the high-pressure connection HA to the control chamber SR. For as long as the injection continues, fuel flows along a valve piston which couples the magnet actuator MA to the pilot valve SV, from a high-pressure zone of the injector I to a low-pressure zone of the injector I. This fuel outflow during the injection is referred to as switching leakage. The occurrence of switching leakage disadvantageously signifies the loss of energy, because compressed fuel flows into the low-pressure zone of the injector.
The injector depicted in FIG. 1 is disclosed e.g. in FIG. 4 of the publication titled “Der BMW Sechszylinder-Dieselmotor mit EU4-Technik” (“The BMW six-cylinder diesel engine featuring EU4 technology”) by Dipl.-Ing. K. Mayer, Dipl.-Ing. W. Neuhäuser and Ing. F. Steinparzer, published at the 25th International Motor Symposium in Vienna, 2004.
As an approach for solving the problem of switching leakage, the control piston SK illustrated in FIG. 1 has a cone K which serves as a stop. When the fuel is injected, the control piston SK and its cone K, moving toward the magnet actuator MA, closes a channel which leads to the discharge throttle AD. Consequently, the pressure in the control chamber SR increases again at least to some extent. The pressure in the control chamber SR increases until it is greater than the pressure in a valve pin chamber DNR containing a valve pin DN. However, if the pressure in the control chamber SR is greater than that in the valve pin chamber DNR, the pilot valve SV opens again. This results in an oscillating motion of the control piston SK and therefore to a continuous opening and closing of the pilot valve SV. Therefore, although the switching leakage is somewhat reduced, the injection is choked due to the continuously repeated closure that is caused by the oscillating motion of the control piston SK and the valve pin DN which is coupled to the control piston SK. Furthermore, the oscillating motion of the valve pin is disadvantageously evident in the injection rate and in the injection jet.
An injector having hydraulic transmission is also disclosed in FIGS. 4 and 5 of the publication titled “A Common-Rail Injection System for High Speed Direct Injection Diesel Engines” by N. Guerrassi and P. Doparz, published in “Society of Automotive Engineers Inc. 1998”. In this case it is proposed that the switching leakage be minimized by means of minimizing the control piston diameter to the minimum possible diameter, specifically to the diameter of the valve pin. This solution has slight advantages with regard to minimizing the switching leakage, but has disadvantages with regard to the switching speed and the dimensioning of the overall injector system.